1. Field of the Invention
The present invention relates to a lithographic apparatus, a substrate table, and a method for enhancing substrate release properties of a substrate table.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
During operation in a lithographic apparatus, a substrate may be clamped to a substrate table that includes a chuck that has a plurality of burls for supporting corresponding parts of a bottom section of the substrate. In order to strive towards optimal fixation during a lithographic process, which is very position sensitive, the chuck is provided with a clamping device, such as an electrostatic clamp or a vacuum system, or geometric clamp surfaces, such as burls.
However, after the lithographic process is finished, the substrate is released from the substrate table for further processing in a different phase of the lithographic process. Due to the activation of the clamping device, the substrate tends to stick to the substrate table, even though the clamping device has been de-activated. This sticking effect may be due to adhesion forces between the bottom section of the substrate and the top section of the supporting burls, as well as to electrostatic forces generated by residual electrostatic charges. Adhesion forces may be generated by material impurities, and roughness imperfectness of the contacting surfaces.
The undesired sticking effect may be counteracted by providing a pressurized gas between the bottom section of the substrate and the top section of the supporting burls. However, this method may not exhibit reliable results, and may cause damage to the substrate and/or burls of the substrate table. Substrates may slip over the burls, thereby causing wear on the burls, which may lead to more sticking forces and fixation performance degradation. In the process of loading a substrate on a sticky burl, overlay and contamination degradation may occur, because horizontal movement of the substrate may be hindered.
JP 2159744 discloses a mechanism for asymmetrically lifting a substrate from the substrate table by means of a substantially centrally located knock-out pin and a swing cam positioned near a peripheral edge of the substrate. The lifting structure aims at peeling the substrate from the substrate table. However, in this peeling process, breakage of the substrate may occur, thereby leading to an undesired reduction in the production yield of substrates.